Many important optical materials exhibit birefringence. Birefringence causes different linear polarizations of light to travel at different speeds through the material. These different polarizations are most often considered as two components of the polarized light, one component being orthogonal to the other.
Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces applied to the material. The induced birefringence may be temporary, as when the material is oscillated, or the birefringence may be residual, as when the material undergoes thermal stress during production.
Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam that traverses a sample of the optical material. If the incident light beam is linearly polarized, the two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). A second convenient expression of retardance is units of phase angle (waves, radians, or degrees), which is the retardance (nm) divided by the wavelength of the light (nm). A “normalized” birefringence for a sample is sometimes computed by dividing the measured retardance magnitude by the thickness of the sample.
The two orthogonal, polarized beam components mentioned above are parallel to two orthogonal axes associated with the optical material, which axes are referred to as the “fast axis” and the “slow axis.” The fast axis is the axis of the material that aligns with the faster moving component of the polarized light through the sample. A complete description of the retardance of a sample along a given optical path requires specifying both the magnitude of the retardance and the relative angular orientation of the fast (or slow) axis of the sample.
The need for precise measurement of birefringence properties has become increasingly important in a number of technical applications, such as the metrology of linear birefringence in optical elements that are used in high-precision instruments employed in semiconductor and other industries.
The prior art, including U.S. Pat. No. 6,473,179, Birefringence Measurement System, hereby incorporated by reference, discloses methods and apparatus for measuring birefringence of a sample using a light beam that is directed through the sample at a normal (zero-degree) incidence angle relative to the surface of the sample. As a result, the determination of the sample's birefringence is “in-plane” or “normal,” meaning that the determination essentially represents the difference between the indices of refraction of two orthogonal axes in a plane of the sample, that plane being normal to the incident light beam. The term “in-plane retardance” or “normal retardance” means the product of the in-plane birefringence and the thickness of the optical sample being measured.
Prior systems, such as that disclosed in the above-noted U.S. Pat. No. 6,473,179, can be designated “point based measurement systems” because the birefringence data is collected for a single point or location in the sample, one point at a time. Such systems are particularly useful with samples having extremely low to mid-range levels of birefringence.
Many display techniques rely on the control of polarized light, and the birefringence of the materials used in systems, such as liquid crystal display (LCD) panels, affect the resulting color and contrast of the image. For liquid crystals and many materials, the extent or magnitude of birefringence is a function of the incident angle of the light under consideration. For example, increasing (from normal) the viewing angle of an LCD panel will increase the birefringence effect on the light emanating from the panel and, without compensation, that increase reduces the perceived quality of the visible light by reducing contrast and/or altering colors. When viewed in normal incidence, the speed of light is affected by two orthogonal refractive indices, nx and ny. The birefringence is a function of the difference in these two properties. When in non-normal incidence, the light is also affected by the third orthogonal refractive index, nz.
Transparent polymer films have been developed for use with LCD panels for the purpose of compensating for the just-noted birefringence variations attributable to viewing angle. In short, these films possess birefringence characteristics that compensate for the birefringence of the LCD panel and thus provide a wide viewing angle without significant loss of contrast or color.
Characterizing the effective birefringence of such films, and other optical materials, in planes that are not normal (zero-degree) to the angle of incidence allows for the optimization, control and analysis of such materials. This birefringence measure can be referred to as “out-of-plane” birefringence. One can consider the notion of in-plane and out-of-plane birefringence in terms of a Cartesian coordinate system. Accordingly, if the normal-incidence light is considered to travel in a direction parallel to the Z-axis of such a coordinate system, the in-plane birefringence occurs in the XY plane of the sample. Out-of-plane birefringence is in a plane not coincident to the in-plane (XY plane) birefringence. Special cases occur in the XZ or YZ plane, which planes are perpendicular to the XY plane. The terms vertical birefringence and “Rth”, are specifically used for those special cases. Rth means the product of the vertical (XZ or YZ) birefringence and the thickness of the optical sample being measured.
Other applications, in addition to the birefringence compensation film example just discussed, may also call for precise determination of out-of-plane birefringence. For example, uniaxial crystals have a unique optical axis (Z-axis). A light beam that propagates perpendicular to this axis experiences the maximal intrinsic birefringence. A light beam that propagates along this axis experiences no intrinsic birefringence. The birefringence on either XZ or YZ plane is the “out-of-plane” birefringence for a light beam propagating along the Z-axis.
In U.S. Pat. No. 7,312,869 (“'869 Patent”), hereby incorporated by reference, there is disclosed a point based method and apparatus for precise measurement of out-of-plane birefringence properties of samples of transparent optical material. Two angled-apart, polarization-modulated light beams are passed through a selected location of the sample optical element. One of the beams is normal or zero-degree incident to the sample surface, and the other beam is oblique to that surface. The characteristics of the beams are detected after passing through the sample, and the information detected is processed to determine the out-of-plane birefringence.
As described in the '869 Patent, the out-of-plane birefringence calculation involves information derived from both the normally incident and oblique beams. Accordingly, in the two-beam approach described there, the single sample location through which the oblique-angled beam penetrates is substantially aligned with, and not significantly different in size than, the location through which the normal-incidence beam penetrates.
As noted above, with many materials the extent or magnitude of birefringence is a function of the incident angle of the light under consideration, and increasing (from normal) the viewing angle of a LCD panel will increase the birefringence effect on the light emanating from the panel. To measure the birefringence of such material at several different angles (thereby to design a suitable compensation film, for example), several measurements must be made over a range of oblique-angle incidences. In a point based system such as that described in the '869 Patent, the sample may be sequentially tilted into several discrete angular positions and at each position the oblique-angled beam is directed through the sample to detect the associated birefringence information for that particular angle. To yield improved results, measurements at multiple angles of incidence can be taken. These multiple measurements are usually done sequentially by mechanically rotating or tilting the sample. These numerous sequential measurements require significant amounts of time to complete.
Irrespective of the particular polarization properties of interest, such as in-plane birefringence only, or both in-plane and out-of-plane birefringence, certain optical-material samples may have configurations and birefringence characteristics that make them amenable to imaging techniques for rapidly collecting across a wide area of the sample the data employed for calculating the sought-after properties. An imaging system for collecting such data across an area of (or the entire area of) a sample will provide high spatial resolution and is particularly useful for low to high levels of birefringence, that is, where the sample is not characterized by extremely low levels of birefringence.